A solid-state light-emitting element such as an LED is attracting attention as a light source for a variety of products since it is smaller, more efficient, and lasts longer.
Examples of products using LEDs as a light source include a luminaire. The number of LEDs used in a luminaire is determined based on a desired brightness. Typically, a number of LEDs are used for a single luminaire. When a number of LEDs are used in a luminaire, the LEDs may be connected in series to one another. In this arrangement, the same current is supplied to the LEDs, and accordingly unevenness in brightness of the LEDs can be suppressed.
For the arrangement in which LEDs are connected in series to one another, if one of the LEDs has an open-circuit failure, current supply is stopped for all of the LEDs, so that the other normal LEDs are not lit as well. In order to address this problem, a technique is known, in which a bypass circuit is connected in parallel to each of the LEDs, and the bypass circuit is turned on when an open-circuit failure occurs in the corresponding LED to thereby supply current to the other normal solid-state light-emitting elements (see, e.g., Japanese Unexamined Patent Application Publication Nos. 2005-310999, 2008-204866, 2003-208993, and 2009-038247).
For such a luminaire, however, excessive current may flow in the other normal LEDs when the bypass circuit is operated. As a result, the normal LEDs may deteriorate or fail.
For example, in the disclosure of Japanese Unexamined Patent Application Publication No. 2009-038247, a bypass circuit is connected in parallel to each of LEDs connected in series, and if an increase in the voltage across an LED having an open-circuit failure is detected, a bypass switch in a corresponding bypass circuit is turned on. In this instance, however, immediately after the bypass switch is turned on, excessive current flows in the other LEDs having no open-circuit failure and in the corresponding bypass circuit. Therefore, in the above disclosure, normal LEDs may deteriorate or fail. In order to prevent the LEDs from deteriorating or failing, the LEDs or the like need to be robust to stress due to such excessive current, causing the cost and size to be increased.
Hereinafter, such a problem will be described in more detail with reference to FIGS. 1A and 1B and FIG. 2.
FIG. 1A is a circuit diagram of a luminaire having bypass circuits. The luminaire shown in FIG. 1A includes: light-emitting elements 103a and 103b connected in series; a bypass circuit 104a connected in parallel to the light-emitting element 103a; a bypass circuit 104b connected in parallel to the light-emitting element 103b; a constant-current circuit 101 for supplying constant current to the light-emitting elements 103a and 103b; and a smoothing capacitor 102 connected between output terminals of the constant-current circuit 101. The light-emitting elements 103a and 103b are, e.g., LEDs.
In this luminaire, if the light-emitting element 103b has an open-circuit failure, the bypass circuit 104b is turned on as shown in FIG. 1B. By doing so, current is supplied to the light-emitting element 103a. As such, the luminaire can prevent that all of the light-emitting elements are lit out when one of them has an open-circuit failure.
Further, in this luminaire, the output voltage VC from the constant-current circuit 101 is monitored, for example, and it is detected that the light-emitting element 103 or 103b has an open-circuit failure if the voltage VC rises above a predetermined voltage.
In this regard, the present inventors have found out that such a luminaire has the following problem.
FIG. 2 shows graphs of the voltage VC versus time and a current I flowing in the normal light-emitting element 103a versus time, in the case where an open-circuit failure occurs.
Before time t1 at which an open-circuit failure occurs, the voltage VC is equal to the sum of forward voltages of the two light-emitting elements 103a and 103b (2×Vf). When an open-circuit failure occurs at time t1, no current flows in the normal light-emitting element 103a and the voltage VC rises. At time t2, the voltage VC rises above a predetermined voltage (i.e., VC>2×Vf). Accordingly, the bypass circuit 104b is turned on.
As the bypass circuit 104b is turned on, the voltage VC decreases up to a voltage equal to the forward voltage Vf of the normal light-emitting element 103a. However, at the moment when the bypass circuit 104b is turned on, the voltage VC is higher than the voltage 2×Vf, and electric charges corresponding to this voltage have been accumulated in the smoothing capacitor 102. Therefore, at the moment when the bypass circuit 104b is turned on, electric charges accumulated in the smoothing capacitor 102, which correspond to a difference voltage (>Vf) between the voltage (>2×Vf) and the forward voltage Vf (i.e., electric charges which correspond to the forward voltage Vf of the light-emitting element 103b having the open-circuit failure) flow in the normal light-emitting element 103a at a burst (from time t2 to time t3).
As such, excessive current may flow in the normal light-emitting element 103a so that the normal light-emitting element 103a may deteriorate or break down. In addition, when excessive current flows in the light-emitting element 103a, the bypass circuit 104a may be erroneously turned on.
In order to suppress excessive current from flowing in the normal light-emitting element 103a, the bypass circuit 104b having a forward voltage equal to the forward voltage of the light-emitting element 103b may be provided. However, this approach may cause another problem in that the bypass circuit 104b has more power loss.
As a technology to suppress such excessive current, there is known a technique in which a voltage drop unit is provided in a bypass circuit (see, e.g., International Publication No. WO 2012/005239). According to this reference, a resistor is provided in a bypass circuit as a voltage drop unit, so that it reduces current flowing immediately after a bypass switch in the bypass circuit is turned on, thereby suppressing stress exerted on LEDs or the like.
In this approach, however, the power loss is continuously generated by the voltage drop unit after connecting two ends of the LED having the open-circuit failure.